Interlock symbol
Also called Logic, Binary logic
An interlock symbol marks a logic function that trips or permits something when conditions are met, such as stopping a pump on low level.
What the interlock symbol means
An interlock symbol marks a logic function that trips or permits something when conditions are met, such as stopping a pump on low level.
How it is drawn
A diamond, usually with an I or a reference to the cause and effect chart inside.
The interlock symbol in each standard
Drawn as ISA 5.1 defines it, as on most US and international legends.
ISO 10628-2 does not cover instruments. On ISO-based P&IDs they are drawn as IEC 62424 PCE requests, or to ISA 5.1 on many international projects.
IEC 62424 has no separate symbol for this. Check the project legend.
German (DIN): Verriegelung. See DIN P&ID symbols.
Where you see it on a P&ID
Where trips, permissives and shutdowns are shown on the P&ID.
Tag codes
The interlock is commonly tagged:
- I
- UZ
Tag formats follow the project numbering procedure. For instruments, the letters follow ISA 5.1, explained on the instrument symbols page.
Related symbols
Frequently asked questions
What does the interlock symbol look like on a P&ID?
A diamond, usually with an I or a reference to the cause and effect chart inside.
What is a interlock used for?
An interlock symbol marks a logic function that trips or permits something when conditions are met, such as stopping a pump on low level. Where trips, permissives and shutdowns are shown on the P&ID.
What tag does a interlock have?
It is commonly tagged I, UZ. The exact tag format comes from the project numbering procedure and the legend sheets.
Is the interlock symbol the same in ISA and ISO?
ISO 10628-2 does not cover instruments. On ISO-based P&IDs they are drawn as IEC 62424 PCE requests, or to ISA 5.1 on many international projects. Companies vary the details, so always check the legend sheets at the front of the P&ID set.
What is the interlock called in German?
Verriegelung. On a German R&I-Fließschema it is drawn to DIN EN ISO 10628-2 and DIN EN 62424.
- In production since 2014
- Thousands of daily users
- On MODEC FPSO projects since 2016